In coal-fired power stations, processes are employed to reduce the amount of SO2 admissions from the combustion of coal. By switching to a lower sulfur coal, such as bituminous or lignite to subbituminous coal, one can lower the SO2 emissions and the amount of fly ash produced per ton of coal burned. Since the fly ash produced from the burning of subbituminous coal produces an ASTM Class C fly ash, the amount of this type of fly ash has been growing throughout the U.S. as coal-fired power plants switch to this type of coal to meet SO2 emission guidelines. Removal and disposal of this particular type of fly ash from coal-fired plants and the fly ash and other byproducts produced from the combustion of other fossil fuels has been costly, and efforts have been devoted to finding uses for the unprocessed raw fly ash which are commercially viable to offset removal costs.
While a large portion of raw fly ash is buried or carted off to landfills, the raw Class C fly ash has been used as backfill due to its ability to set up, for instance, within 30 minutes. However, this quick-setting characteristic of the fly ash is offset by the relatively low strength of the cement produced in this manner. For instance, in a seven-day test of raw Class C fly ash, the compressive strength is only 2,925 psi when run as per ASTM C 989 testing protocol when blended at 50% OPC/50% fly ash. This compressive strength is not generally acceptable as a good enough strength gain to use at Grade 120 slag replacement factors of 50%. It is desirable for high-strength cementitious products to have a compressive strength exceeding 5,000 psi in 7 days versus straight OPC which attains around 4,400-4,800 psi in 7 days, running the same ASTM C 989 test. Thus, this type of raw Class C fly ash cannot be used to replace cement at 50% or greater and attain a high-strength cementitious product that exhibits strengths that exceed Grade 120 slag strengths in 7 and 28 days.
It will be noted that cementitious products having compressive strength of greater than 5,000 psi in 7 days and over 7,000 in 28 days as measured by the C 989 test in general exceeds Grade 120 slag performance and is useful in a wide variety of high-strength cement applications. In fact, cements having a Grade 100 slag rating as measured by this test or better have proven to be satisfactory in some applications. Unfortunately, raw fly ash cannot approach such slag performance and is, therefore, relegated to patching and other backfill applications or much lower cement replacement factors than the normal 50% replacement factor.
Regardless of the strength characteristics of raw Class C fly ash, in the past, raw fly ash has been used in cements but at replacement concentrations of less than 25% by weight in which the raw fly ash replaces 25% or less of Ordinary Portland Cement (OPC). Regardless of strength considerations, this low replacement percentage renders raw fly ash not as desirable versus a high cement replacement-factor material as a practical matter.